This invention relates generally to the development of latent electrostatic images which are formed on the photoconductive surface of electrophotographic members. More particularly, the invention provides a method and apparatus for effecting such development by transfer rather than using conventional electrophoretic liquid development techniques.
The formation of a latent electrostatic image on the surface of a photoconductive member by electrophotographic means is well known to the art. Likewise, the development of such electrostatic image to render same visible also is well known to the art. The electrophotographic technique of image reproduction involves placing a uniform electrostatic surface charge potential on a photoconductive surface, exposing the charged photoconductive surface to a radiation pattern so as to form a latent electrostatic image and then developing the latent electrostatic image by depositing thereon finely divided usually pigmented electroscopic particulate material referred to in the art generally as toner. The toner particles are attracted to those areas of the surface retaining the electrostatic charge in proportion to the field strengths of the respective incremental areas defining the pattern. The toned image either may be fixed or fused to said surface as by heat or other suitable means or may be transferred to a secondary support medium such as paper and then fixed thereupon if desired or necessary.
In some known electrophotographic copying or duplicating machines the photoconductive member is in the form of a drum which rotates relative to a plurality of processing stations. For high speed copying it has been found that the photoconductive surface should be in a flattened or planar disposition at the time of exposure in order to ensure complete focussing of the original document or article being copied. Consequently, it has been found advantageous to employ a photoconductive member in the form of an endless belt or web mounted for rotational movement across at least two spaced rollers and defining a pair of generally parallel reaches.
Regardless of whether the photoconductor member is in the form of a drum or of an endless belt mounted on rollers, the latent electrostatic charge image carried thereon can be developed into a visible image by using methods categorized as so-called dry methods, for example, cascade development and magnetic brush development, and so-called wet methods involving employment of a dispersion or suspension of electroscopic pigmented toner particles in an insulating liquid. In liquid development the liquid containing the suspended particles is applied to the photoconductive surface to cover same in both the charged and uncharged areas. Under the influence of the electric field associated with the latent electrostatic image charge pattern, the suspended electroscopic particles migrate through the liquid toward the charged portions of the surface and separate from the suspending liquid. The migration of charged toner particles is due to the phenomenon called electrophoresis and such migration results in the deposition of the toner particles on the photoconductive surface in image configuration. The quantity of the toner particles adhering at any one location is directly proportional to the strength of the electrical field of the latent charge image at that location. The particles actually travel through the insulating liquid suspending medium toward the surface upon which they are deposited and sufficient liquid is needed to enable such migration. The electrophoretic process depends greatly upon the toner particle mobility in the insulating medium.
Electrophoretic development generally has been accomplished by flowing the liquid toner developer over the image bearing surface by immersing the image surface in a bath of such developer. Another method of development presents the developer liquid on a smooth surfaced roller and relative nonsynchronous movement of the image carrying surface and the applicator roller is effected. Some development methods include dynamically flowing a stream of the suspended particles past the image bearing surface at a station where a predetermined path is defined for such fluid flow. It is further known electrically to assist the migration of the toner particles toward the photoconductive surface employing development electrodes.
In U.S. Pat. No. 4,025,339 issued on May 24, 1977 to M. R. Kuehnle there is described an electrophotographic film that is capable of being imaged with quality and gray scale as good as, if not better than, that achieved by photographic techniques. The film comprises an inorganic coating of microcrystalline material that is bonded onto a conductive substrate. The inorganic coating may comprise a layer of radio frequency sputtered cadmium sulfide having a thickness of about 2,000 Angstroms to 2 microns. The conductive substrate may comprise a layer of indium tin oxide or other conductive material having a thickness of about 500 .ANG. deposited on a sheet of stable polyester plastic about 5 microns thick. A latent electrostatic image formed on the film may be developed using a liquid toner.
In order to make the fullest use of the exceptional properties of the electrophotographic film described in the above noted patent, especially for high speed duplicating or copying machine applications, there is a need for a simple yet efficient technique for developing the formed latent electrostatic image with a liquid toner.
The inorganic photoconductive coating formed according to the teachings of the referenced patent is characterized particularly by its ordered microcrystalline orientation. The individual crystallites comprising the coating are density packed and all oriented generally vertically to the receiving surface with the result, among others, that the coating is electrically anisotropic. The lateral resistivity of the surface of the photoconductive coating is unusually high while the transverse resistivities are substantially lower. Conductivity through the coating upon exposure to actinic radiation is substantial. The charges held on or near the surface do not readily migrate laterally but are retained relatively immovable. Each crystallite of the coating has its own electrical field when charged. Each field attracts toner particles independently of all other fields.
The practical resolution capabilities of the electrophotoconductive coating of the referenced patent for the purpose of electrophotographic reproduction of images depends to a considerable extent upon the minimum size of the toner particles available and the utility as well as the capability of presenting to the electrostatic image toner particles of such size and in sufficient quantity to achieve the sought after toner density.
The employment of development processes using liquid toner suspensions enables the use of finer particle toners than are used with dry methods which in turn enables the achievement of resolution results commensurate with the capability of the patented photoconductive coating. Ultra fine particles are available only via liquid toner suspension.
Difficulty has been encountered in achieving uniform toning over the width of the latent charge image. Uniform toning demands uniformity of the toner particle suspension fed to the photoconductive surface. Agitation of the toner suspension within the applicator tank was considered essential to proper development. With agitation there develops undesired turbulence which often continues during the feeding of the toner suspension to the photoconductive surface to be toned. The amount of toner delivered to the toning location and hences to the latent image must be carefully controlled.
Other problems encountered during the conventional electrophoretic process of developing electrostatic latent charge images include spillage of toner and the insulating liquid medium either from its container or from the applicator roller; the necessity of and difficulty in removing excess toner from the photoconductive surfaces; difficulties in establishing a uniform precise toning gap and, as well, the proper electrical bias voltage across the gap and the lack of versatility as to the type and concentration of the toner particle which can be employed.
It is important to provide for distributive uniformity in the suspension presented to the latent image. There is a tendency for the suspended toner particles to agglomerate into large clumps or accumulations of particles. If the relative ratio of particle to carrier liquid becomes too great, uneven toning results. The flow pattern of the developer must not be turbulent.
Employing known techniques it has been found difficult to define and to maintain uniformity of the toning gap, that is, at the toning location. Additionally too much insulating liquid may be delivered to the toning gap and hence must be dealt with to meet environmental standards as to contamination.
As briefly mentioned conventional electrophoretic toning processes employ relatively dilute suspensions of toner particles in an insulating liquid.
A most serious impediment in liquid toning processes resides in the time duration needed for the toner particles to move through the dispersant liquid toward the photoconductive surface requiring many seconds, much less than the duration sought for high speed operation. A faster process for toning has been sought.
Another difficulty experienced in liquid toning processes involves the removal of excess carrier or insulating liquid. Further, an insufficient number of toner particles may be delivered to the latent image at the toning station. Thus incomplete toning may result unless the duration of toning is extended and/or multiple toning passes are effected. Often there occurs unacceptable reduction in optical density, failure evenly or uniformly to tone all portions of the latent image, migration of toner particles preferentially to certain select areas of the latent image and random washing of toner. Electrophoretic migration of the toner particles through the insulating liquid medium has been found to enhance the formation of so-called Benard convection cells. These cells may be attracted preferentially over the toner particles to the surface of the photoconductor and deny access to the photoconductive surface by toner particles otherwise attractable thereto, tiny voids being formed in the toned image.
Conventional electrophoretic toning methods have been electrically assisted by use of development electrodes and precise toning gaps. Establishment and maintenance of these gaps require tolerances to be maintained which considerably increase the cost of the equipment. The necessity for the toner particles to traverse a considerable distance through the liquid carrier generally increases the voltage required for electrical toning assistance. This in turn requires many precautions to be taken, such as in electrically isolating connections, etc.
Evaporation of the insulating liquid attendant with the use of low viscosity suspensions as well as the loss of liquid due to spillage, loss by excessive application to the belt, carryover due to the excess, etc. loss due to liquid creep, whipping due to the relative high speed operation and undesired layering of flow and turbulence at the delivery location are problems encountered during conventional electrophoretic toning processes which give rise to the desire for a different and improved developing process. Layering causes differential adhesion to the particles to the surface areas.
Depletion of the toner suspension generally has been rapid so that frequent replenishment of the toner suspension at the toning station has been required. Thus the provision of a supply tank for fresh toner supply vessel and attendant feed means generally is mandatory auxiliary equipment.
It would be highly desirable for maximum space utilization and cost reduction if the necessity for replenishment of the toner suspension during the normal run life would be avoided; however, so long as electrophoretic toning processes are used, dilute solutions generally will be employed and replenishment factors such as provision of secondary reservoirs, tanks, conduits, valving, etc. apparently are required.
In addition to cost reduction, both on construction, fabrication, assembly and maintenance, improvement of the effective toning process, the achievement of desirable optical density and resolution characteristics commensurate with the ability of the electrophotographic member to perform should be the goal sought by way of improvement in the development process.